Abstract
BackgroundThe filamentous fungus Trichoderma reesei (anamorph of Hypocrea jecorina) produces increased cellulase expression when grown on cellulose or its derivatives as a sole carbon source. It has been believed that β-glucosidases of T. reesei not only metabolize cellobiose but also contribute in the production of inducers of cellulase gene expression by their transglycosylation activity. The cellulase hyper-producing mutant PC-3-7 developed in Japan has enhanced cellulase production ability when cellobiose is used as the inducer. The comparative genomics analysis of PC-3-7 and its parent revealed a single-nucleotide mutation within the bgl2 gene encoding intracellular β-glucosidase II (BGLII/Cel1a), giving rise to an amino acid substitution in PC-3-7, which could potentially account for the enhanced cellulase expression when these strains are cultivated on cellulose and cellobiose.ResultsTo analyze the effects of the BGLII mutation in cellulase induction, we constructed both a bgl2 revertant and a disruptant. Enzymatic analysis of the transformant lysates showed that the strain expressing mutant BGLII exhibited weakened cellobiose hydrolytic activity, but produced some transglycosylation products, suggesting that the SNP in bgl2 strongly diminished cellobiase activity, but did not result in complete loss of function of BGLII. The analysis of the recombinant BGLII revealed that transglycosylation products might be oligosaccharides, composed probably of glucose linked β-1,4, β-1,3, or a mixture of both. PC-3-7 revertants of bgl2 exhibited reduced expression and inducibility of cellulase during growth on cellulose and cellobiose substrates. Furthermore, the effect of this bgl2 mutation was reproduced in the common strain QM9414 in which the transformants showed cellulase production comparable to that of PC-3-7.ConclusionWe conclude that BGLII plays an important role in cellulase induction in T. reesei and that the bgl2 mutation in PC-3-7 brought about enhanced cellulase expression on cellobiose. The results of the investigation using PC-3-7 suggested that other mutation(s) in PC-3-7 could also contribute to cellulase induction. Further investigation is essential to unravel the mechanism responsible for cellulase induction in T. reesei.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0420-y) contains supplementary material, which is available to authorized users.
Highlights
IntroductionThe filamentous fungus Trichoderma reesei (anamorph of Hypocrea jecorina) produces increased cellulase expression when grown on cellulose or its derivatives as a sole carbon source
The filamentous fungus Trichoderma reesei produces increased cellulase expression when grown on cellulose or its derivatives as a sole carbon source
Single‐nucleotide point mutation in bgl2 and the resulting amino acid substitution Comparative genomic analysis of the genes encoding BGLII in KDG-12 and PC-3-7 identified a singlenucleotide difference at position 1298, where guanine was substituted by thymine
Summary
The filamentous fungus Trichoderma reesei (anamorph of Hypocrea jecorina) produces increased cellulase expression when grown on cellulose or its derivatives as a sole carbon source. It has been believed that β-glucosidases of T. reesei metabolize cellobiose and contribute in the production of inducers of cellu‐ lase gene expression by their transglycosylation activity. The cellulase hyper-producing mutant PC-3-7 developed in Japan has enhanced cellulase production ability when cellobiose is used as the inducer. Several transcription regulators that control cellulase gene expression have been isolated from T. reesei These include Xyr, a key activator of cellulase and hemicellulase expression [9, 10], the additional activator ACEII [11] and ACEIII [12], the repressor ACEI [13, 14], and the β-glucosidase activator BglR [15]. The wide domain carbon catabolite repressor Cre, which represses cellulase gene expression during growth on glucose, has been reported [16,17,18]
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